A gas bag which defines a part of a side-impact protection means is known from U.S. Pat. No. 5,322,322. This head gas bag is configured hose-shaped and secured by its side ends to the A-pillar and to the B-pillar. In the folded-up condition, the gas bag extends under a covering along the A-pillar over the roof frame up to the B-pillar so that it is incorporated in the vehicle substantially bow-shaped. The gas bag comprises a special fabric which, on inflation of the gas bag, produces a substantial shortening in length of the same. On deployment the gas bag opens the covering and extends more or less linearly from one securing end to the opposite one.
On impact of the head of a vehicle occupant, the known gas bag is not supported by the side window, which is usually destroyed in any case due to side impact, it instead being tensioned between its side ends.
However, the special fabric makes the known gas bag relatively expensive. In addition to this, with this fabric only a limited shortening of the gas bag in the longitudinal direction can be achieved on inflation.